The Postman's Hat, All-Night Mask, Blast Mask, Keaton Mask, Romani's Mask, Kafei's Mask, and Couple's Mask can be learned about at Blast Mask Quest, Anju & Kafei Quest, and Romani & Cremia Quest. The other 17 will be talked about here.
As soon as you talk to the Mask Salesman with the ocarina of time, he will teach you the Song of Healing. Learning it will function like playing it to heal yourself and then you will turn into a human and get the Deku Mask.
You need the Lens of Truth to follow Darmani to his grave. He will be in his grave when you enter even if you didn't follow him there. To get the Goron Mask, you must play the Song of Healing in front of Darmani. Playing the Song of Healing right in front of him won't work unless you already used the Lens of Truth at least once since you last entered the grave room. Every time you dupe a bottle over the Goron Mask, Darmani will be in his grave again so you can get it back by playing the Song of Healing there again. The arrangement of the platforms to the Lens of Truth cave is different on each day but those platforms are gone in the spring.
To get the Zora Mask, you have to first push Mikau to shore then play the Song of Healing. You will get the Zora Mask after you play it whether or not you listened to his final message. Every time you dupe a bottle over the Zora Mask, you can get back the Zora Mask exactly the same way as you got it the first time, by rescuing Mikau then healing him.
Fierce Deity's MaskEdit
The only way to get the Fierce Deity's Mask is to complete all the moon trials then talk to the kid sitting in front of the tree who takes you into the final boss room. You keep the Fierce Deity's Mask after you beat the game but you only lose it when you play the Song of Time if you haven't beaten the game since you got it. You can only wear the Fierce Deity's Mask in boss rooms. If you dupe a bottle over the Fierce Deity's Mask, you can get it back by giving away all the non-transformation masks again but will only keep it if you don't play the Song of Time before you beat the game.
You can only talk to the soldier in the rock circle on the path to Ikana Canyon when you're using the Lens of Truth. If you give him a red potion when you're talking to him, he will give you the Stone Mask or if you already have it, a blue rupee. You can only give him a red potion once in a cycle. Wearing the Stone Mask also makes enemies ignore you. Wearing it also makes all the pirate guards not see you except for the 2 in the hookshot room other than the one at the entrance to that room.
Great Fairy MaskEdit
If you return the stray fairy in Clock Town to the fairy fountain there while you have the Deku Mask, you will get the Great Fairy Mask. Stray fairies in the same area as you will go towards you when you're wearing that mask when they're not in a bubble. Once you returned the stray fairy in Clock Town, you can over and over dupe a bottle over the Great Fairy Mask and get it back just by rentering the fairy fountain until you play the Song of Time
Guru Guru is only in the laundry pool on the nights of the first and second day. He never gives you anything when you talk to him in deku form. He will only give you something the first time in the cycle that you talk to him there in a different form than deku form. The first time you talk to him in a different form than deku form, he will give you the Bremann mask if you don't have it or nothing if you already have it.
If you march around with the Bremann Mask in the cucco shack until all 10 chicks grow up, Grog will give you the Bunnyhood or if you already have it, a red rupee. The grownup chickens don't turn back into babies until you play the Song of Time so you can't do that challenge again in the same cycle.
Don Garo's MaskEdit
If you throw a rock sirloin onto the high ledge in mountain village, you will get the Don Garo's Mask or if you already have it a purple rupee, as long as there's a goron on that ledge. That goron is never there in the spring. He starts off there in the winter but once you give him a rock sirloin, he leaves and doesn't come back until you play the Song of Time. The way you're supposed to get it is by goron spike rolling into the part of the chandelier it's in. It's always in the same one and is supposed to be out of reach until the chandelier starts spinning. There are 2 ways to make it start spinning, lighting all the thin torches in the goron shrine and shooting the centre piece with a fire arrow. There's also a glitch getting the rock sirloin without the chandelier spinning by hitting the chandelier to make it swing so that the rock sirloin won't be out of reach the next time you hit it and another glitch of hovering right behind it then rolling to make it fall out. The chandelier resets back to its original position with the rock sirloin back where it started when you play the Song of Time.
Mask of ScentsEdit
The deku shrine challenge isn't on until after you release the deku princess right in front of the Deku King in the royal chamber which is the only place you can release her. If you catch the deku princess in a duped bottle then leave the area and come back, there will be another deku princess there. They will kick you out of the deku palace when you're in the royal chamber in any form other then deku form until you release the Deku Princess then they won't kick you out no matter what form you're in. If you win the deku shrine challenge, you will get the Mask of Scents or if you already have it, a red rupee. Once you win the deku shrine challenge, you can't take it again until you play the Song of Time and release the Deku Princess again.
Circus Leader's MaskEdit
Of the times the Milk Bar is open, the show is only on from 10:00 pm to 5:00 am on the first and second day. Only the first time in each cycle you do the show you get something. You get the Circus Leader's Mask for doing it whether or not you already have it. After you do the show, you can do it again just by standing under the correct spotlight for your form but you don't get anything from it so you can't get back the Circus Leader's Mask in the same cycle as you dupe over it.
Mask of TruthEdit
Each time you talk to the man at the entrance of the Swamp Spider House the first time since you beat it in the current cycle, he will give you the Mask of Truth, or if you already have it, nothing. Once you already talked to him since you beat it in the current cycle, you can't get anything from him so if you dupe over the Mask of Truth after you get it from him in the same cycle, you can't get it from him again in the same cycle.
If you heal Kamaro, he will give you the Kamaro Mask or nothing if you already have it. He's only there from midnight to 6 AM on each day. He disappears once you heal him. After you play the Song of Time, he starts being there again during those hours.
To get the Captain's Hat You're supposed to play the Sonata of Awakening in front of Captain Keeta then beat him before he goes too far up the path, then he will remove the fire around the chest that has the Captain's Hat in it. If you open that chest when you already have the Captain's Hat, you will get a recovery heart. If you get the Captain's Hat with the Stop Time glitch, the chest won't open so you can get it back in the same cycle after you dupe over it. You can also glitch yourself into getting it without first doing the fight by wierdshoting to that chest then clipping through the fire ring. Each night, there are 3 stall children walking around a different grave. You can make them break open a grave they're walking around by talking to them while wearing the Captain's Hat and that grave will have a hole under it. You can also clip into one of those holes when the grave is there as discussed later. All graves come back when you play the Song of Time. The grave of the first night takes you to an iron knuckle you have to beat to make a heart piece chest appear. The grave of the second night takes you to an iron knuckle you have to beat to learn the Song of Storms. The grave of the final night takes you to Dampe's house where you have to beat a big poe to make a bottle chest appear. The door beside Captain Keeta which is locked from the outside takes you to the same place but you can also recoil clip through that door.
The music box house will always be locked before you play the Song of Storms in the spring water cave. Once you play the Song of Storms there, water flows along the river that starts there and the gibdos walking around the Music Box House die. Also the music box house starts playing. When it's playing Pamela will come out every 2 minutes and when you make an explosion near the door. She also goes back into the house when she sees you. The door is only unlocked when she's out of the house and you have to get the Gibdo mask by healing the father in that house. After you heal him, he will give you the Gibdo Mask or nothing if you already have it. After that, the house is always unlocked. After you get it then dupe over it in the same cycle, you can't get it back in the same cycle. You can also recoil clip into the music box house. The spring water cave resets when you play the Song of Time.
You can pay 10 rupees to do the horse race as long as Epona is in that area with you. The race can be done any time during the day but not at night. If you win the race, you will get the Garo mask or if you already have it, a bottle refilled with Milk or if all the normal bottles are full, nothing. Even if you get the Garo Mask then dupe over it in the same cycle, you can still get it back in the same cycle by doing the horse race again. If you won the alien fight, that will not stop you from being able to do the race any time you normally could do it because the fences in Milk Road and the race track are only rearranged during the night of the second day.
The Giant's Mask is in the treasure chest in Stone Tower Temple just before the boss room. If you open that chest when you already have the Giant's Mask, you will get a recovery heart. The Giant's Mask can only be worn in the Stone Tower Temple boss room.